1. Field of the Invention
The present invention relates to object based graphic systems and, in particular, discloses a method and apparatus for providing transparency in an object based rasterized image prior to displaying that image on either a video display or by means of printing.
2. Description of the Related Art
Most object based graphics systems utilise a frame store to hold a pixel based image of the page or screen. The outlines of the objects are calculated, filled and written into the frame store. For two-dimensional graphics, objects which appear in front of other objects are simply written into the frame store after the background object, thereby replacing the background on a pixel-by-pixel basis. This is commonly known in the art as "Painter's algorithm". Images are calculated in object order, from the rearmost object to the foremost object. However, real time image generation for raster displays can be efficiently achieved by calculating the images in raster order. This means that each scan-line must be calculated as it is reached. This requires that the intersection points of each scan line with each object outline are calculated and subsequently filled.
Although it is possible, with a line buffer rather than a page buffer, to use the Painter's algorithm within a scan-line to fill the objects, such an approach requires multiple writing of pixels at high speed to achieve real-time operation.
Furthermore, it is generally required to provide the visual effect of "transparency" in which some objects are rendered (displayed) in a transparent fashion (e.g. an object visible but in shadow, or a distant object viewed through a window). This is generally achieved using "Painter's algorithm" by averaging the colours of the objects that overlap to form the transparent portion of the image.